1. Field of the Invention
The invention relates to a technique of controlling a rotational motion of a motor based on a signal indicative of the rotational state of the motor, and more particularly to a technique of controlling the motor in a manner resistant to an additive noise.
2. Description of the Related Art
Typically, various kinds of machines and apparatuses using motors need control of the respective motors. For fulfilling such a need, a technique has been widely practiced in which a rotational state signal is generated indicative of the rotational state of the motor, and in which the rotational motion of the motor is controlled based on the generated rotational state signal.
An example of such a machine is an image forming apparatus of an electrophotographic type such as a laser printer. The image forming apparatus is configured to include a polygon mirror for use in deflecting a laser beam exiting a semiconductor laser, a photosensitive drum on which an electrostatic latent image is formed with the laser beam exiting the polygon mirror, etc. These elements are each categorized into a movable element. Such a movable element requires a precise control of the rotational speed of the movable member. The image forming apparatus may include other elements each having the same requirement.
One example of a technique of controlling the rotational state of a motor has been commonly carried out, in which a frequency generator is employed to output a frequency signal indicative of the rotational speed of the motor, and in which the rotational motion of the motor is controlled through feedback control, based on the frequency signal outputted from the frequency generator.